1. Field of the Invention
The present invention relates to a process for separating acetonitrile as the by-product in a crude olefinically unsaturated nitrile such as acrylonitrile and methacrylonitrile produced in the ammoxidation of propylene or isobutylene. More particularly, it relates to a process for efficiently using acetonitrile separated as a fuel or incinerating the acetonitrile without deteriorating the quality of the olefinically unsaturated nitrile.
2. Description of the Prior Art
The reaction gas obtained in the ammoxidation of propylene or isobutylene is fed to a quenching column where unreacted ammonia is removed. Then the reaction gas quenched is counter-currently contacted with absorbing water in an absorption column, and a reaction waste gas containing unreacted propylene or unreacted isobutylene, carbon monoxide, carbon dioxide, nitrogen, etc. is discharged from the top of the absorption column while an aqueous solution containing acrylonitrile or methacrylonitrile, acetonitrile, hydrogen cyanide and other impurities is withdrawn from the bottom of the absorption column. The aqueous solution thus obtained is fed to an extractive distillation column, to the upper part of which water is introduced as an extractant. From the top of the extractive distillation column are recovered acrylonitrile or methacrylonitrile, hydrogen cyanide and a small amount of water while an aqueous solution mainly containing acetonitrile is withdrawn from the bottom of the extractive distillation column. The aqueous acetonitrile-containing solution is introduced to a stripping column and from the bottom of the stripping column is recovered water substantially not containing acetonitrile and recycled to the absorption column and the extractive distillation column as the absorbing water and the extractant, respectively. As a result, acetonitrile, low boiling point impurities such as acetone, acrolein, oxazole, propionitrile and a small amount of hydrogen cyanide, and a small amount of water are withdrawn from the top of the stripping column. The aqueous solution containing acetonitrile withdrawn from the top of the stripping column is usually employed as a fuel or subjected to incineration. In this case, the concentration of acetonitrile in the aqueous acetonitrile-containing solution is usually as low as about 2% by weight and in order to impart natural combustibility to the aqueous solution, the concentration of the acetonitrile in the aqueous solution is required to be at least about 10% by weight. Accordingly, the stripping column requires a device for enriching acetonitrile, and for this purpose is generally employed a distillation column equipped with a reflux condenser. Such a device, however, increases the operational cost and in addition, the amount of heat for evaporation water in the aqueous solution and heating the evaporated water employed in incineration, i.e. the amount of an auxiliary fuel required is increased. According to Japanese Patent Publication No. 36892/1970, part of the bottoms liquid in the extractive distillation column is recycled as the absorbing water to the upper part of the absorption column and is contacted with a waste gas in the absorption column, whereby the acetonitrile in the bottoms liquid of the extractive distillation column is evaporated into the waste gas in the absorption column and then the resulting waste gas from the absorption column is subjected to incineration. In this method vapor-liquid contact is conducted by contacting an aqueous solution of a low concentration, i.e. about 2,500 ppm of acetonitrile from the bottom of the extractive distillation column with the waste gas in the absorption column. It is preferred that the temperature of water flowing down in the absorption column is low in order for the water to absorb the reaction exhaust gas therein. However, when the temperature is low, the evaporation of acetonitrile and other low boiling impurities into the waste gas in the absorption column is insufficient. As a result, the concentrations of acetonitrile and other low boiling impurities in the aqueous solution which is fed to the extractive distillation column from the bottom of the absorption column are increased, that is, acetonitrile and other low boiling impurities are recycled from the bottom of the extractive distillation column to the upper part of the absorption column and the extractive distillation column. Thus, the separation of acetonitrile and other low boiling impurities must be conducted under more severe conditions and the concentrations of acetonitrile and other low boiling impurities in the crude olefinically unsaturated nitrile from the top of the extractive distillation column are disadvantageously increased. Further, according to this conventional process, substantially the entire amount of acetonitrile and other low boiling impurities produced in the ammoxidation of propylene or isobutylene cannot be withdrawn from the waste gas from the absorption column and as a result, the remaining acetonitrile is withdrawn from the top of the stripping column and subjected to incineration. Thus, the enriching device for acetonitrile provided in the stripping column is still necessary, the operational cost cannot be completely reduced and the auxiliary fuel used in an incinerator is still required. According to Japanese Patent Application No. 149,620/1975, a waste gas having a little higher temperature from the top of the absorption column is blown from the bottom of a stripping column succeeding to an extractive distillation column to conduct stripping of acetonitrile in an aqueous solution from the bottom of the extractive distillation column, and the gas discharged from the top of the stripping column is incinerated. Also, in this process an aqueous solution of a low concentration, i.e. about 2,300 ppm of acetonitrile from the bottom of the extractive distillation column is contacted with a waste gas from the absorption column in a plate column. Furthermore, the waste gas for incineration from the absorption column contains about 10% by volume to about 30% volume of water and the amount of an auxiliary fuel employed for incineration of acetonitrile as a fuel is disadvantageously increased.